FIELD OF THE INVENTION
The invention relates to nonlinear-optically active copolymers, to polyadducts produced from them, and to the use of the latter for nonlinear-optical media, i.e., in electrooptical and photonic components.
Electrooptical and photonic components are important elements in nonlinear optics and in optical information technology. They are planar waveguide structures whose function can be altered by an electrical voltage. They comprise modulators, Mach-Zehnder modulators, tunable and switchable directional couplers, wavelength filters, including tunable wavelength filters, and polarization-modifying waveguide components. Their construction is described, for example, by R. C. Alferness in T. Tamir "Guided-Wave Optoelectronics", Springer-Verlag Berlin Heidelberg 1988, pages 145 to 210, and in K. J. Ebeling "Integrierte Optoelektronik", 1st edition, Springer-Verlag Berlin Heidelberg 1989, pages 152 to 162.
Components of this kind can be produced using highly anisotropic inorganic crystals which have a high 2nd-order susceptibility.
In the past, organic materials and polymers having high 2nd-order susceptibilities have also been developed. They feature considerable advantages in terms of their preparation and their use in electrooptical and photonic components. Polymers having nonlinear-optical (NLO) properties are known from the literature; in this context see, for example: S. R. Marder, J. E. Sohn, G. D. Stucky "Materials for Nonlinear Optics", ACS Symposium Series, Vol. 455 (1991), pages 128 to 156, R. A. Norwood et al. in L. A. Hornak "Polymers for Lightwave and Integrated Optics", Marcel Dekker, Inc., New York 1992, pages 287 to 320, and G. J. Ashwell, D. Bloor "Organic Materials for Nonlinear Optics", Royal Society of Chemistry, Cambridge 1993, pages 139 to 155 and 332 to 343.
An overview of current problems in the development of materials having pronounced NLO properties was recently published by T. J. Marks and M. A. Ratner in Angew. Chem. 107 (1995), pages 167 to 187. In addition to the requirements that have to be set for nonlinear-optical chromophores, reference is also made to the problems in developing polymeric matrices for the embedding or binding of chromophores, and their orientation-stable alignment.
In order for such polymers, which are provided with covalently bonded or dissolved NLO chromophores, become nonlinear-optically active and have a high 2nd-order susceptibility, the chromophores must be oriented in an electrical field (in this respect, see: J. D. Swalen et al. in J. Messier, F. Kajzar, P. Prasad "Organic Molecules for Nonlinear Optics and Photonics", Kluwer Academic Publishers 1991, pages 433 to 445). This normally takes place in the region of the glass transition temperature, where the mobility of the chain segments of the polymers allows orientation of the NLO chromophores. The orientation obtained in the field is then frozen in by cooling. The 2nd-order susceptibility .chi..sup.(2) that is achievable here is proportional to the spatial density of the hyperpolarizability .beta., to the ground-state dipole moment .mu..sub.o of the chromophores, to the electrical poling field, and to parameters which describe the distribution of orientation following the poling process (in this respect, see: K. D. Singer et al. in P. N. Prasad, D. R. Ulrich "Nonlinear Optical and Electroactive Polymers", Plenum Press, New York 1988, pages 189 to 204).
Great interest attaches to compounds combining high dipole moment with high values of .beta.. Consequently, investigation has focused in particular on those chromophores which consist of conjugated n electron systems that carry an electron donor at one end and an electron acceptor at the other end and are covalently bonded to a polymer: for example, to polymethyl methacrylate (U.S. Pat. No. 4,915,491), polyurethane (EP-A 0 350 112), or polysiloxane (U.S. Pat. No. 4,796,976).
One particular problem of said polymer materials having NLO properties is the relaxation of the oriented chromophore units and thus the loss of NLO activity. At present, this relaxation is still preventing the production of electrooptical components with long-term stability that are deployable technically.